Silica sand (SiO2, quartz) is widely used as an aggregate in the metal casting industry for the production of molds and cores. It is used for both “greensand” (sand bonded with water and clay) and for chemically bonded sand. A variety of inorganic and organic chemical binders are used including phenolic urethane, furan, epoxy-acrylic, and ester-cured phenolic.
The binders are mixed with the sand and the mixture is compacted in tooling to take the shape of the desired mold or core, then the binder hardens and bonds the sand grains together. The mold and core components are then assembled into a mold package and metal is poured into the package and fills the internal cavities in the shape of the desired casting. The heat from the liquid metal, especially in the case of ferrous alloys with melting points in excess of 1100° C., starts to decompose the binder and heat the sand. As the silica sand heats, thermal expansion occurs. This expansion is relatively linear until the temperature reaches approximately 570° C. when the crystal structure of the sand grains transforms. This structural transformation is accompanied by rapid isothermal expansion followed by a period of thermal shrinkage up to around 980° C. when another crystal structure change occurs with more thermal expansion.
It is believed that these rapid volumetric changes in the sand grains develop mechanical stresses in the layers of sand near the casting surface that can lead to cracking of the mold or core surface which is in contact with the hot molten liquid metal in the mold. The molten liquid metal can flow into these cracks and form veins or fins on the casting surface. These are undesirable and require time and effort to remove. In critical applications with small internal cored passageways, the vein can extend across and block the passageway. Examples of these critical castings are engine blocks and heads with water jackets that could be blocked by the veins in locations that are difficult to detect and even more difficult to remove.
Other types of aggregates can also be used to produce “sand” molds and cores including naturally occurring zircon, chromite, olivine, and man-made ceramic and other aggregates. These have lower expansion rates with no phase changes and a much reduced tendency to form veining defects, but are also much more expensive.
Sand additives have been used with silica sand to reduce the veining tendency. These sand additives typically fall into three main categories based on their mechanism of activity.
The first category consists of “low expansion aggregates” such as a 90:10 mix of silica and zircon sand, which has a lower expansion value than silica alone. In addition to the naturally occurring aggregates, man-made aggregates like ceramic (mullite) beads, aluminum-silicate “microspheres”, or fused silica can be used.
The second category consists of “organic cushioning materials” such as wood flour, dextrin and starch. When mixed with the silica sand, they occupy some volume between the sand grains. Thus, when molten metal is poured into the mold, the heat from the molten metal quickly burns out the extra organic material. The volume previously occupied by the organic material can then provide a “cushion” or space for the expansion of the sand, thus reducing the buildup of stresses in the sand.
The third category of sand additives consists of “fluxes” that react with the surface of the sand grains to chemically change the surface layer of the sand and the resulting expansion characteristics of the sand. Examples of such fluxes are iron oxides, both hematite (Fe2O3) and magnetite (Fe3O4), which have long been used as sand additives. Other flux-type sand additives include titanium oxide (TiO2) and lithia (Li2O) containing materials such as spodumene. It has also been demonstrated that the use of a combination of several different flux type additives may have a beneficial effect. This is particularly true when hematite is used with other additives.
The existing categories of sand additives can reduce veining in castings, but all three categories of sand additive have some important disadvantages. The low expansion aggregates tend to be expensive compared to silica sand and need to be used at relatively high levels (greater than 10 percent based on sand). The organic cushioning materials tend to add to the total amount of gas produced by the mold or core when exposed to liquid metal and can significantly reduce mold/core strength when used at levels above about 1 percent. The flux-type sand additives are currently the most widely used additives, but they also have some drawbacks. For instance, iron oxides, when used above about 2 percent by weight based on the sand (BOS) can cause increased metal penetration and can reduce mold/core strength when used at higher levels. The lithia bearing spodumenes are expensive and typically are used in higher levels, e.g. 4-8 percent by weight based on the sand (BOS).